1. Field of the Invention
The invention relates generally to frequency measuring apparatus operative in the presence of spurious frequency components, and more particularly to measurement of frequency coded angle data as in a Doppler Navigation system or the like.
2. Description of the Prior Art
The so-called Doppler Navigation system (Doppler ILS) is known per se, and is variously described in patents and other technical literature. For example, basic forms of radio beacons which provide the typical commutated array signal transmission format are described in U.S. Pat. Nos. 3,626,419 and 3,670,337.
In a Doppler radio navigation system (sometimes called Doppler ILS) in connection with which the embodiments are to be described, there is an established beacon signal transmission format involving commutation of a first radio frequency in turn to each of a plurality of linear array antenna elements so as to simulate constant velocity scanning of the array (with a scan time of T), whereby a main-bearing signal is transmitted which, as "seen" by a remote receiving station of the system, undergoes a Doppler frequency shift proportional to the sine of the angle subtended by the receiver normal to the axis of the array. Also included is simultaneous transmission of a reference signal of a second radio frequency (offset from the first) against which the receiver is able to derive a Doppler beat waveform from the said main-bearing signal, the beat having a frequency indicative of the angle of the said remote receiving station.
One of the most serious problems arising in such systems as the aforementioned is the problem of multipath signals. The remote receiving station ideally would receive only a direct path signal, but in the practical situation spurious signals are also extant at the receiver as a result of reflections from nearby object (including the ground).
The manner in which the present invention deals with this and other prior art problems will be understood as this description proceeds.